Class AB digital to analog converter/line driver

ABSTRACT

A differential line driver includes first, second, third and fourth cascode transistors connected in parallel, wherein drains of the first and third transistors are connected to a negative output of the differential line driver, and wherein drains of the second and fourth transistors are connected to a positive output of the differential line driver. First, second, third and fourth switching transistors are connected in series with corresponding first, second, third and fourth cascode transistors and driven by a data signal. First and second compound transistors inputting a class AB operation signal at their gates, wherein the first compound transistor is connected to sources of the first and second switching transistors, and wherein the second compound transistor is connected to sources of the third and fourth switching transistors.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation of U.S. patent application Ser. No. 10/158,193, Filed: May 31, 2002, Titled: CLASS AB DIGITAL TO ANALOG CONVERTER/LINE DRIVER, inventors: Mulder et al., which is a Continuation-in-Part of application Ser. No. 10/153,709, Filed: May 24, 2002, Titled: DISTRIBUTED AVERAGING ANALOG TO DIGITAL CONVERTER TOPOLOGY, Inventors: MULDER et al.; and is related to application Ser. No. 10/158,774, Filed: May 31, 2002; Titled: ANALOG TO DIGITAL CONVERTER WITH INTERPOLATION OF REFERENCE LADDER, Inventors: MULDER et al.; application Ser. No. 10/158,595, Filed: May 31, 2002, Titled: HIGH SPEED ANALOG TO DIGITAL CONVERTER, Inventor: Jan MULDER; and application Ser. No. 10/158,773, Filed: May 31, 2002, Inventor: Jan MULDER; Titled: SUBRANGING ANALOG TO DIGITAL CONVERTER WITH MULTI-PHASE CLOCK TIMING, Inventors: van der GOES et al., all of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to line drivers, and more particularly to high-speed, low-distortion line drivers.

[0004] 2. Related Art

[0005]FIG. 1 shows a conventional output driver cell of a line driver currently employed in (Gigabit) Ethernet products. Each cell includes two differential pairs, enabling tri-state operation. Transistors M1 a through M1 d are cascodes, implemented using thick-oxide transistors. Transistors M3 a and M3 b implement the tail current sources of the two differential pairs, each providing a current I_(BIAS). Transistors M2 a through M2 d are switches (typically thin-oxide transistors) that control to which output terminal the bias current I_(BIAS) is sent. More specifically, when V_(switch1) and V_(switch3) are logical “1”, and V_(switch2) and V_(switch4) are “0”, the differential output current I_(OUT) equals −2 I_(BIAS). When V_(switch1) and V_(switch3) are “0”, and V_(switch2) and V_(switch4) are “1”, I_(OUT) equals 2 I_(BIAS). When V_(switch1) through V_(switch4) are all “1”, I_(OUT) equals zero. (In other words, the digital signal, or data signal, activates the switching transistors M2 a-M2 d.) A more detailed description of a conventional line driver can be found in commonly assigned U.S. Pat. No. 6,259,745.

[0006] V_(BIAS) is a DC bias voltage that biases the tail current transistors M3 a, M3 b to an analog amplifier mode. The switches M2 a-M2 d send current to either the “+” or the “−” terminal of the output cell, which is a tri-state operating cell. The cell outputs either 2 I_(BIAS), 0, or −2I_(BIAS). To output zero current, while operating the cell in class B mode, gates of switches M2 a-M2 d are switched to ground, and no current appears at the output. Due to the charge injected at node {circle over (1)}, the potential at the gate of M3 changes, resulting in distortion. Thus, there is unwanted modulation of the DC bias on the gate of the tail current transistors M3 a, M3 b.

[0007] As noted above, when I_(OUT) has to be zero, V_(switch1) through V_(switch4) switch to “0”. Unfortunately, switching off all four switches M2 a-M2 d results in significant distortion of the output signal I_(OUT). The cause of the distortion is explained by FIG. 2, which shows half of a line driver output cell. The distortion occurs when all four switches M2 a-M2 d are switched to “0”. In that case, node {circle over (2)} goes to ground potential. Through the parasitic capacitance C_(p), charge is injected onto node {circle over (1)}. In general, the bias voltage V_(BIAS) is generated by a current-biased diode, which has a finite output impedance modeled by R_(BIAS). Furthermore, the parasitic capacitance C_(p,bias) associated with the bias voltage V_(BIAS) source and transistor M3 is quite large. As a consequence, the charge injected onto node {circle over (1)} causes the voltage on node {circle over (1)} to drop. It settles back slowly due to the finite voltage source impedance and the large parasitic capacitance connected to node {circle over (1)}. This results in modulation of the tail currents of the differential pairs, and therefore, in modulation of the amplitude of I_(OUT), in other words, unwanted distortion.

[0008] I_(OUT) (in differential mode)=I_(OUT+)−I_(OUT−)·I_(OUT) is the differential output signal current. Its magnitude depends on the symbol to be transmitted and varies from −40 mA to 40 mA (in 1000BT, 100TX mode), from −100 mA to 100 mA in 10BT mode). In Class AB mode, I_(COMMON) _(—) _(MODE)=(I_(OUT+)−I_(OUT−))/2 varies from 20 mA to 10 mA, depending on the symbol to be transmitted. Thus: I_(COMMON) _(—) _(MODE)=20 mA (in Class-A mode).

[0009] I_(COMMON) _(—) _(MODE) varies from 20 mA to 10 mA (in Class-AB mode), hence the maximum saving of current is 10 mA. When I_(COMMON) _(—) _(MODE) switches from 20 mA to 10 mA, a glitch is seen that eventually settles to constant value—i.e., producing unwanted distortion.

SUMMARY OF THE INVENTION

[0010] The present invention is directed to a line driver that substantially obviates one or more of the problems and disadvantages of the related art.

[0011] There is provided a differential line driver includes first, second, third and fourth cascode transistors connected in parallel, wherein drains of the first and third transistors are connected to a negative output of the differential line driver, and wherein drains of the second and fourth transistors are connected to a positive output of the differential line driver. First, second, third and fourth switching transistors are connected in series with corresponding the first, second, third and fourth cascode transistors and driven by a data signal. First and second compound transistors inputting a class AB operation signal at their gates, wherein the first compound transistor is connected to sources of the first and second switching transistors, and wherein the second compound transistor is connected to sources of the third and fourth switching transistors.

[0012] In another aspect of the present invention there is provided a differential line driver including first and second half-cells, the half-cells cross connected to positive and negative differential outputs, each half-cell including first and second cascode transistors connected in parallel. First and second switching transistors are connected in series with the first and second cascode transistors and driven by a data signal. A first compound transistor inputs a class AB operation signal at its gate and connected to sources of the first and second switching transistors.

[0013] In another aspect of the present invention there is provided a differential line driver includes first and second half-cells, the half-cells cross connected to positive and negative differential outputs, each half-cell including first and second cascode transistors are connected in parallel. First and second switching transistors are connected in series with the first and second cascode transistors and driven by a voltage. A tail current transistor inputs a bias voltage at its gate and connected to sources of the first and second switching transistors, wherein a sum of charge injection at the gate of the tail current transistor is substantially zero during switching.

[0014] In another aspect of the present invention there is provided a class AB line driver including first and second half-cells cross connected to positive and negative differential outputs, each half-cell including first and second cascode transistors connected in parallel. First and second switching transistors are connected in series with the first and second cascode transistors. A tail current transistor inputs a bias voltage at its gate and connected to sources of the first and second switching transistors, wherein the bias voltage spikes last less than a clock cycle during switching for Gigabit Ethernet operation.

[0015] In another aspect of the present invention there is provided a programmable line driver including a plurality of cells, each cell selectively controlled by class AB operation signal and each cell including first and second half-cells cross connected to outputs of opposite polarity, each half-cell including first and second parallel cascode transistors. First and second switching transistors are each connected in series with the first and second parallel cascode transistors. A compound transistor inputs a bias signal at its gate and connected to sources of the first and second switching transistors, the compound transistor switched to class AB operation by the class AB operation signal, wherein same polarity outputs of the cells are added.

[0016] In another aspect of the present invention there is provided a differential line driver including a plurality of cascode transistors connected in parallel and to corresponding polarity outputs of the differential line driver. A plurality of switching transistors are connected in series with corresponding cascode transistors. A plurality of compound transistors input a class AB operation signal at their gates and connected in series with corresponding switching transistors.

[0017] In another aspect of the present invention there is provided a differential line driver including first and second half-cells, the half-cells cross connected to positive and negative differential outputs, each half-cell including first and second cascode transistors connected in parallel. First and second switching transistors connected in series with the first and second cascode transistors. A variable effective channel length transistor inputting a signal that changes its effective channel length at its gate and connected to sources of the first and second switching transistors.

[0018] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0019] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:

[0021]FIG. 1 illustrates a conventional line driver;

[0022]FIG. 2 illustrates a half cell of a conventional line driver;

[0023]FIG. 3 illustrates a line driver of the present invention;

[0024]FIG. 4 illustrates a half cell of the line driver of the present invention;

[0025]FIG. 5 illustrates additional detail of the circuit of FIG. 4

[0026]FIG. 6 illustrates current and voltage waveforms at various nodes of FIG. 5;

[0027]FIG. 7 illustrates an output current waveform;

[0028]FIG. 8 illustrates a bias voltage waveform at node 1;

[0029]FIG. 9 illustrates a voltage waveform at node n3 of FIG. 5;

[0030]FIG. 10 illustrates a voltage waveform at nodes n3 a and n3 b of FIG. 5;

[0031]FIGS. 11 and 12 illustrate a multi-cell driver of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0033] Low-power operation is very important for Gigabit Ethernet chips. The line driver is an important contributor to the overall power consumption. (The line driver, or transmitter, is frequently a digital-to-analog converter (DAC), but will be referred to as “line driver” herein.) Therefore, class AB or class B operation, instead of class A operation, are alternatives for decreasing the power consumption of the line driver.

[0034] Line drivers frequently have class B operating mode. Unfortunately, the distortion of operating in that mode is higher than desired. A new circuit technique for implementing a low-distortion class AB line-driver for (Gigabit) Ethernet applications, is described herein, allowing significant reduction in power consumption, while preserving sufficient line driver linearity. In order to reduce power consumption of the line driver, it is desirable to reduce the quiescent current (idle current) I_(IDLE).

[0035] Class A operation typically biases drivers to a certain (fairly large) quiescent current I_(IDLE), e.g., I_(IDLE)=40 mA. In class AB operation, the quiescent current I_(IDLE) is much smaller, or about 20 mA. For class B operation, the output cell is only turned on for when it is driving something, otherwise it is turned off. The idle current I_(IDLE) for class B operation is essentially zero. As noted above, I_(COMMON) _(—) _(MODE)=10 mA in class AB mode. Hence the I_(IDLE)—the output current during idle mode in the best case scenario—is (10 mA+10 mA)=20 mA (10 mA from the positive terminal, 10 mA from the negative terminal). In class A operation, I_(COMMON) _(—) _(MODE)=20 mA and I_(IDLE)=40 mA (20 mA from the positive terminal, 20 mA from the negative terminal)

[0036] The circuit of the invention prevents charge injection to node {circle over (1)} from occurring. To accomplish this, the simple tail current transistors M3 a, M3 b shown in FIG. 1 are replaced by “compound” transistors 301, 302, shown in FIG. 3. Transistors M5 a and M5 b are switches that control the effective channel length L_(EFF) of the compound transistors 301, 302. When V{overscore (_(CLASSAB))} is “0”, L_(EFF) is determined by the channel length of the transistors M3, M4 and M6. When V{overscore (_(CLASSAB))} is “1”, M5 effectively short circuits the transistor M4, and L_(EFF) is determined mainly by the channel length of the transistors M3 and M6. Therefore, when V{overscore (_(CLASSAB))} switches from “1” to “0”, L_(EFF) increases, which causes the tail current source, and hence the power consumption, to decrease. The output cell switches to class B operation when I_(OUT) needs to be zero, by switching V_(switch1) through V_(switch4) to “1” and V{overscore (_(CLASSAB))} to “0”.

[0037] Further with reference to FIG. 3, FIG. 3 shows a differential output cell, comprising two half-cells, a left half-cell and a right half-cell. The output cell includes four cascode transistors M1 a, M1 b, M1 c, and M1 d, which are arranged in parallel. Drains of the cascode transistors M1 a and M1 c are connected to the negative polarity output, and drains of the cascode transistors M1 b and M1 d are connected to the positive polarity output of the differential cell. Gates of the cascode transistors M1 a-M1 d are driven by V_(CASC). V_(CASC) is a DC voltage of approximately 1 volt, and output voltage is about 1.8 V in the idle state. The transistors M1 a through M1 d are implemented using thick-oxide transistors. Note that the drains of the transistors M1 a-M1 d are connected to a primary coil of a transformer (not shown in the figures). A center tap of the transformer is connected to a 1.8 V supply

[0038] The four cascode transistors M1 a-M1 d are connected in series with corresponding switch transistors M2 a-M2 d, sources of the cascode transistors M1 a-M1 d connected in series with drains of the switch transistors M2 a-M2 d. Gates of the switch transistors M2 a-M2 d are driven by corresponding inputs V_(switch1) through V_(switch4). The transistors M2 a through M2 d are switches (typically thin-oxide transistors, driven by about 1-1.2 volts) that control to which output terminal the bias current I_(BIAS) is sent—i.e, a data signal drives the gates of the transistors M2 a through M2 d.

[0039] Sources of the switch transistors M2 a, M2 b are tied together and to a “drain” of the compound transistor 301. The compound transistor 301 includes three internal transistors M3 a, M4 a, M6 a connected in series. Gates of the internal transistors M3 a, M4 a, and M6 a are connected to a bias voltage V_(BIAS). The source of the last transistor, M6 a, is connected to ground. A fourth internal transistor M5 a is connected across M4 a. A gate of the internal transistor M5 a is driven by the V{overscore (_(CLASSAB))} signal. When the V{overscore (_(CLASSAB))} signal goes to a logical one, the transistor M4 a is shorted, and the effective channel length L_(EFF) of the compound transistor 301 is reduced. The impedance of the transistor M5 a is low when it is on.

[0040] The compound transistor 302 operates in a similar manner to compound transistor 301.

[0041] The table below shows the tri-state class AB operation of the circuit of FIG. 3: M2a M2b M2c M2d output

0 1 0 1 I_(OUT+) 1 1 0 1 0 I_(OUT−) 1 1 1 1 1 I_(OUT+), I_(OUT−) 0

[0042]FIG. 4 shows a half-cell of FIG. 3, with the parasitic capacitances shown as circuit elements. As shown on FIG. 4, node {circle over (1)} has C_(P,bias), connected to ground (a bias parasitic capacitance). Parasitic capacitance C_(P1) is between node {circle over (1)} and node {circle over (2)}, parasitic capacitance C_(P2) is between node {circle over (1)} and node {circle over (3)}, and parasitic capacitance CP₃ is between node {circle over (1)} and node {circle over (4)}.

[0043] The transistors M2 a-M2 d are used as switches, and the compound transistors 301, 302 are used as analog amplifiers. Typically, field effect transistors are used as the transistors illustrated in FIG. 3.

[0044] The aspect ratios W/L of individual transistors comprising the compound transistor 301 (or 302) are such that voltage at the node of V_(BIAS) (node {circle over (1)}) is not affected during switching, and idle current I_(IDLE) is reduced by about fifty percent. Because there are no substantial interruptions, or spiking of the bias voltage V_(BIAS), the output current is much cleaner, and shows less distortion. Note that since V{overscore (_(CLASSAB))} changes the aspect ratio of the compound transistors 301, 302, instead of changing L_(EFF), it is equally possible to change effective channel width, or both Leff and the effective channel width.

[0045] Further with reference to FIG. 4, when the signal V{overscore (_(CLASSAB))} at the gate of M5 a goes to zero, the potential at node {circle over (4)} goes down, the potential at node {circle over (2)} goes up, and the potential at node {circle over (3)} goes up. Thus, although more parasitics are involved, the net result of the charge injection is zero. C_(P1), C_(P2) and C_(P3) of FIG. 4 deliver opposite polarity charges into node {circle over (1)}. Thus, although node {circle over (1)} has slow settling time, there is never any net charge injection, since the charge injection cancels out.

[0046] The compound transistor 301 behaves as if it is a single transistor, even though in actuality it includes at least four transistors. The aspect ratio of the compound transistor 301 depends on the state of the switch M5 a, since the effective channel length L_(EFF) varies depending on the input to M5 a:

Aspect Ratio=W/L _(EFF)

[0047] By changing L_(EFF), the effective aspect ratio W/L changes.

[0048] Thus, FIG. 4 explains why the new circuit implementation does not suffer from significant output current distortion. When the output cell switches to class-AB operation, the voltages on nodes {circle over (2)} and {circle over (3)} will increase. Through parasitic capacitances C_(p1) and C_(p2) a positive charge injection onto node {circle over (1)} occurs. However, the voltage on node {circle over (4)} will decrease and through C_(p3) a negative charge injection onto node {circle over (1)} occurs. If the dimensions of transistors M3, M4, M6 are carefully chosen, the positive and negative charge injections cancel each other, leaving the voltage on node {circle over (1)} undisturbed. This enables low-distortion class AB operation.

[0049]FIG. 5 shows additional detail of the structure of the half-cell, including dimensions of transistors used in one embodiment of the present invention. For example, the transistor M1 a has a width of 3.22 μm, and a length of 0.4 μm. The multiplicity factor m=20 refers to the actual number of transistors M1 a in each half-cell, i.e., in this case, 20. FIG. 5 also shows four NMOS transistors MC2 a, MC2 b, MC1 a, MC1 b, whose sources and drains are connected to V_(SS), so that they function as capacitors. With the transistors of FIG. 5, the aspect ratio is about 160 when V{overscore (_(CLASSAB))} is HIGH, and about 80 when V{overscore (_(CSSAB))} is LOW. In other words, for FIG. 5, when V{overscore (_(CLASSAB))} is HIGH, the aspect ratio=3.22*20*2/(0.28+0.52)=161.

[0050]FIGS. 6 and 7 illustrate a simulation of the output current I_(OUT) of the present invention. As may be seen from FIGS. 6 and 7, the output current I_(OUT) is very clean, showing only a small spike upon transition when operating in Gigabit Ethernet and 100TX mode.

[0051]FIG. 8 shows the potential at node {circle over (1)} (V_(BIAS1)), and particularly illustrating the small spike of approximately 20-25 mV upon transition. The three curves shown in FIG. 8 represent operation at three different temperatures, 125° C., SS process for the top curve, 75° C., TT process for the middle curve, and 25° C., FF process for the bottom curve. Here:

[0052] SS—slow NMOS transistor, slow PMOS transistor.

[0053] TT—typical NMOS transistor, typical PMOS transistor.

[0054] FF—fast NMOS transistor, fast PMOS transistor.

[0055] As discussed above, such a small spike of 20-25 mV for a very short duration (i.e., substantially less than a clock cycle) results in a much cleaner output current, and very low distortion. Another way to look at it is to consider an area under the curve (i.e., spike energy) of each spike in FIG. 8, which is very low relative to overall pulse energy.

[0056]FIG. 9 shows the voltage at node n3 of FIG. 5 at three different temperatures and processes, 125° C. (SS process), 75° C. (TT process), and 25° C. (FF process). Similarly, FIG. 10 shows simulated voltages at nodes n3 a and n3 b of FIG. 5, at the three different temperatures discussed above. As may be seen from these figures, the voltage spikes on V_(BIAS) due to charge injection are very small, enabling low distortion operation.

[0057]FIGS. 11 and 12 illustrates the entire line driver of the present invention that is comprised of a total of 40 output cells (24 cells are shown in FIG. 12) connected in parallel to each other, so that their output currents sum. Each “rectangle” in FIG. 12 corresponds to the circuit (cell) shown in FIG. 3. FIGS. 11 and 12 also illustrate that the output cells of the line driver can be grouped together so as to program their output current. For example, only the cells of Group 1 may be activated, resulting in an output current of 5 mA. When all the groups are activated by V{overscore (_(CLASSAB))} being on, the total output current is 40 mA (i.e., 8×5 mA current of the single group).

[0058] Note that the 40 pair differential line driver can operate at 40 mA maximum output current at 1000 Base T mode, or TX mode, with 2 V_(PP) (peak to peak) output. It can also operate at 100 mA maximum output current in 10 Base T mode, 5 V_(PP) output.

[0059] The quiescent current I_(IDLE), when in class AB operation, can be controlled in several different ways. First, the dimensions (aspect ratios) of the individual transistors that comprise the compound transistor, determine the quiescent current obtained within each output cell. Secondly, more elaborate compound transistors are possible that allow programmability of the quiescent current of the DAC output cells. Thirdly, the switches M5 of all output cells comprising the complete line driver do not have to be controlled by only one signal V{overscore (_(CLASSAB))}. Using different switch signals (e.g., V{overscore (_(CLASSAB1))} . . . V{overscore (_(CLASSAB8))}) for different groups, or subsets, of output cells allows programmability of the overall quiescent current I_(IDLE) of the entire line driver.

[0060] The class AB line driver of the present invention can, in principle, be used in any application where a digital to analog converter (DAC) is used as a line driver.

[0061] It will be appreciated that the various aspects of the invention as further disclosed in U.S. patent application Ser. No. 10/158,193, Filed: May 31, 2002, Titled: CLASS AB DIGITAL TO ANALOG CONVERTER/LINE DRIVER, inventors: Mulder et al.; application Ser. No. 10/153,709, Filed: May 24, 2002, Titled: DISTRIBUTED AVERAGING ANALOG TO DIGITAL CONVERTER TOPOLOGY, Inventors: MULDER et al.; application Ser. No. 10/158,774, Filed: May 31, 2002; Titled: ANALOG TO DIGITAL CONVERTER WITH INTERPOLATION OF REFERENCE LADDER, Inventors: MULDER et al.; application Ser. No. 10/158,595, Filed: May 31, 2002, Titled: HIGH SPEED ANALOG TO DIGITAL CONVERTER, Inventor: Jan MULDER; and application Ser. No. 10/158,773, Filed: May 31, 2002, Inventor: Jan MULDER; Titled: SUBRANGING ANALOG TO DIGITAL CONVERTER WITH MULTI-PHASE CLOCK TIMING, Inventors: van der GOES et al., all of which are incorporated by reference herein, may be combined in various ways, or be integrated into a single integrated circuit or product.

[0062] It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A differential line driver comprising: first, second, third and fourth cascode transistors driven by a first voltage, wherein drains of the first and third transistors are connected to a first output of the differential line driver, and wherein drains of the second and fourth transistors are connected to a second output of the differential line driver; first, second, third and fourth switching transistors connected in series with corresponding first, second, third and fourth cascode transistors and each driven by a corresponding data signal; and first and second compound transistors inputting a control signal to control output currents of the compound transistors, wherein the first compound transistor is connected to sources of the first and second switching transistors, wherein the second compound transistor is connected to sources of the third and fourth switching transistors, and wherein the first compound transistor includes first, second and third field effect transistors (FET's) connected in series with each other and with sources of the first and second switching transistors, and a fourth FET connected across the second FET, a gate of the fourth FET driven by the class AB operation signal.
 2. The line driver of claim 1, wherein the control signal is a class AB operation signal.
 3. The line driver of claim 1, wherein the second compound transistor includes fifth, sixth and seventh FET's connected in series with each other and with sources of the third and fourth switching transistors and an eighth FET connected across the sixth FET, a gate of the eighth FET driven by the control signal.
 4. The line driver of claim 1, wherein an effective channel length of the first compound transistor changes based on the control signal.
 5. The line driver of claim 1, wherein an effective channel length of the first and second compound transistors is variable.
 6. A differential line driver comprising: first, second, third and fourth cascode transistors driven by a first voltage, wherein drains of the first and third transistors are connected to a first output of the differential line driver, and wherein drains of the second and fourth transistors are connected to a second output of the differential line driver; first, second, third and fourth switching transistors connected in series with corresponding first, second, third and fourth cascode transistors and each driven by a corresponding data signal; and first and second compound transistors inputting a control signal to control output currents of the compound transistors and their effective channel widths, wherein the first compound transistor is connected to sources of the first and second switching transistors, and wherein the second compound transistor is connected to sources of the third and fourth switching transistors.
 7. The line driver of claim 6, wherein total charge injected at a bias terminal of the first compound transistor is substantially zero.
 8. A differential line driver comprising: first and second half-cells, the half-cells cross connected to positive and negative differential outputs, each half-cell including: first and second cascode transistors; first and second switching transistors connected in series with the first and second cascode transistors and each driven by a corresponding data signal; and a compound transistor inputting a control signal to change its output current and its aspect ratio, and connected to sources of the first and second switching transistors.
 9. The line driver of claim 8, wherein the compound transistor includes first, second and third FET's connected in series with each other and with sources of the first and second switching transistors, and a fourth FET connected across the second FET, a gate of the fourth FET driven by the class AB signal.
 10. A differential line driver comprising: first and second half-cells, the half-cells cross connected to positive and negative differential outputs, each half-cell including: first and second cascode transistors; first and second switching transistors connected in series with the first and second cascode transistors and each driven by a corresponding data signal; and a compound transistor inputting a control signal to change its output current and its an effective channel length, and connected to sources of the first and second switching transistors.
 11. The line driver of claim 10, wherein total charge injected at a bias terminal of the first compound transistor is substantially zero.
 12. A differential line driver comprising: a plurality of cascode transistors connected to corresponding polarity outputs of the differential line driver; a plurality of switching transistors connected in series with corresponding cascode transistors; and a plurality of compound transistors inputting a class AB operation signal at their gates to control their common mode output current to range from 20 mA to 10 mA, and connected in series with corresponding switching transistors.
 13. A differential line driver comprising: a plurality of switching transistors connected to corresponding polarity outputs of the differential line driver; and a plurality of compound transistors inputting a class AB operation signal at their gates to control their common mode output current to range from 20 mA to 10 mA, and connected in series with corresponding switching transistors. 